The present invention relates to gypsum scale inhibiting compounds and their use in flue gas desulfurization (FGD) systems.
The formation of calcium sulfate and scale inhibition has been studied in connection with desalination, petroleum, geochemistry, and cooling water processes. Because high amounts of calcium sulfate and calcium sulfite are major reaction products in the removal of sulfur dioxide, recent research has focused on the inhibition of scale formation in FGD systems.
In general, wet FGD scrubbers remove sulfur from flue gas by contacting the gas with water in an absorber. The dirty water is slurried in a reaction tank, thickened and then hardened for land fill. In a wet FGD scrubbing process, limestone (CaCO.sub.3) or lime (CaO or Ca(OH).sub.2) reagent reacts in the absorber with sulfur dioxide containing flue gas to produce calcium sulfite hemihydrate (CaSO.sub.3.1/2 H.sub.2 O) and calcium sulfate, present as gypsum (CaSO.sub.4. 2H.sub.2 O). These solids plate out or form scale within the FGD unit. For example, scale has been found in the absorber, mist eliminators, slurry reaction tank, transfer lines, pump parts, valves, reheaters and induced-draft fans of FGD scrubbers.
Calcium sulfite hemihydrate scale is typically soft and generally may be removed by either reducing slurry pH or hydroblasting the unit with water. Gypsum scale, however, is much harder and is very difficult to remove. Gypsum scale is very prominent in limestone systems since limestone's low solubility requires a large excess amount of limestone reagent for sulfur dioxide removal. In addition, gypsum scale is severe in forced-oxidation FGD units where sulfite is converted to sulfate by bubbling oxygen through the absorber slurry.
Mist eliminators are also an integral part of wet scrubber FGD systems. They are used to remove drift or mist from the flue gas as it leaves the scrubber. The liquid mist droplets are comprised of reagent recirculating slurry which is usually made of reagent limestone or lime, containing 5 to 15 percent suspended solids. The slurry is also saturated with calcium sulfate dihydrate (gypsum) and calcium sulfite upon contact with flue gas.
When the drift particles are removed from the gas stream by the mist eliminator, the solid phase of the drift tends to deposit on the mist eliminator baffle slats and the liquid portion gathers into larger droplets and falls back into the slurry tank. This allows the build up of solids which if allowed to remain, degrades the performance of the mist eliminators making removal difficult.
Performance of the mist eliminator for removing small drift particles is important. If unsatisfactory, solids will build up in the down-stream flue gas ducts, in the bottom of the stack, or on the reheater which will impede heat transfer. As the deposits build up on the mist eliminators the gap between the slats is decreased, causing a reduced amount of open area for the flue gas to pass. This causes an increase in gas velocity away from the design velocity resulting in smaller particles not impacting the eliminator blades and not being removed. Furthermore, the deposits will plug the mist eliminator to the point that the induced draft fan cannot remove the flue gas at the required rate, causing the system to be shut down for cleaning.
Hardening of the deposits on the mist eliminators is thought to be caused by a number of reasons. These include supersaturated water chemistry in the drift particles with respect to calcium and sulfate, undissolved lime or limestone particles which deposit on the eliminators from the drift then slowly dissolve causing localized high calcium or pH conditions, or high calcium sulfite slurry drift depositing and slowly oxidizing to gypsum.
To prevent these solids from building up on the mist eliminators, scrubbers have mist eliminator wash systems. Most of the mist eliminators are washed intermittently, as opposed to continuously, due to water requirements and restrictions. Even with the continuous wash systems deposits still build up in many systems, usually forming a hard scale, requiring the system to eventually be shut down for cleaning. Sometimes poor quality wash water is used which only compounds the problem. Accordingly, means for inhibiting gypsum scale formation especially in FGD mist eliminators are needed in the art.
Generally, inhibition is accomplished by dispersing the particulate matter which forms the scale in the medium containing such matter, or controlling precipitate forming ions under conditions in which precipitation would be expected.
U.S. Pat. No. 4,566,973 discloses reducing or inhibiting formation and deposition of scale in a water system with copolymers of an acrylic acid and a substituted acrylamide. The copolymers have a molecular weight in the range of about 1,000 to 50,000. This method, however, only inhibits the formation of phosphate scale.
U.S. Pat. No. 3,880,620 discloses scrubbing flue gases from a blast furnace for iron production with an aqueous medium containing a compound comprising an organo-phosphonate and a water soluble acrylic acid polymer.
However, good calcium phosphate scale inhibitors have not necessarily been found to be good calcium sulfate scale inhibitors, and vice versa. Phosphate scale forms at solution pH higher than the typical operating pH of 4.5 to 6.5 of an FGD scrubber slurry where gypsum formation is common.
Hexamethylene diamine tetra (methylene phosphonic acid) salts ("phosphonates") and low molecular weight polyacrylates, such as those known as Dequest, Goodrite K-752 and Acrysol LMW-10X are known to influence the precipitation of well defined seeded gypsum crystals.
A commonly known phosphonate, Dequest 2054, is a potassium salt of hexamethylene diamine (25% active acid) tetra (methylene phosphonic acid) having the following formula: ##STR1## Polyacrylates are known to have the following general formula: ##STR2## Where n is an integer sufficiently large in number so as to achieve a weight average molecular weight between about 1,000 and about 50,000, and M is H, an alkali metal, an alkaline earth metal, a transition metal, NH.sub.4, a C.sub.1 -C.sub.4 alkyl and the like, or a mixture thereof.
In FGD systems, gypsum scaling conditions vary between a saturation index value of about 1 and about 10. But, little is known about the potential influence of these compounds in bulk, high concentration gypsum solutions.
New compounds and compositions of matter which economically inhibit or preclude the formation of gypsum scale in FGD systems would provide a noticeable advance in the art, be widely accepted, and enjoy extensive commercial success. It is therefore an object of the present invention to provide compounds and compositions of matter that would inhibit the formation of gypsum scale in bulk gypsum solutions, and specifically in FGD systems. It is further an object of the present invention to provide a process for inhibiting the formation of gypsum scale in FGD systems by carrying out the FGD process in the presence of gypsum scale inhibitors. Other objects apparent from the specification are also contemplated. It is to be understood, however, that these objectives are not to be considered a limitation of the present invention, the spirit and scope of which is delineated in the appended claims.